I just discovered Illustration Magazine, via Boing Boing, and their archive includes PDF versions of their older issues and thumbnails of their more recent issues.
As they say:
Illustration is a beautiful, educational, and scholarly magazine devoted to the history of American illustration art. Published quarterly and printed in full color, each issue features the highest quality printing, photography and color reproductions of original art available anywhere.
For those with an interest in popular culture, commercial art and design, publishing history, comic books, paperbacks, pulp magazines, or collecting original art, Illustration is the best source for new information on the illustrators of the past.